1. Field of the Invention
The present invention relates generally to methods and apparatus for generating plasma and ion and to the use thereof. The invention has particular utility with respect to plasma and ion sources used for industrial processes such as plasma treatment, plasma enhanced chemical vapor deposition (PECVD) and plasma etching of substrates and will be described in connection with such utility, although other utilities are contemplated such as for use as electric propulsion devices for space applications.
2. Description of Related Art
Plasma and ion sources are usefully applied in a number of processes including: Plasma enhanced chemical vapor deposition (PECVD), reactive ion etching, plasma surface modification and cleaning, increasing the density of evaporated or sputtered films and assisting a reactive evaporation or sputtering process. Of growing interest is the application of these processes to larger substrates such as flexible webs, photovoltaic panels and architectural and vehicle glass.
Several plasma and ion sources are commercially available and many more have been disclosed. Commercially available plasma and ion sources include: Hollow cathode plasma sources, gridded ion sources, end hall ion sources, closed drift type ion sources (both extended acceleration channel and anode layer types) and impeded anode types such as the APS Pro available from Leybold Optics USA. While successfully applied to small substrate applications like semiconductors or optical filters, they are less effective in processing wide substrate applications. This is primarily due to the use of point electron sources for beam creation and neutralization. Point electron source technologies such as filaments, heated low work function materials and hollow cathodes are difficult to linearly extend. Consequently, the ion and plasma sources that rely on these point electron sources have difficulty producing the uniform linear beams required for large area substrates. In the case of anode layer ion sources, while a uniform ion beam is emitted over a long source length, these sources suffer from lack of ion density and the beam can be difficult to neutralize for long sources.
Therefore, there is a need for a uniform, linear plasma or ion source that can be readily extended to wide substrates. This ideal linear source should also not require a delicate or expensive electron source, such as filaments or LaB6 cathodes, and should be capable of operating over a wide process pressure range. This source should also be physically compact, economical and should produce a dense, efficient plasma beam.
Another important criterion for the ideal linear plasma source is the ability to maintain a continuous PECVD process without excessive coating of the source. In prior art sources such as Lopata et al. in U.S. Pat. No. 5,904,952, the source electrode is exposed to the precursor gases and substrate in the process chamber. The result is that while useful coatings may be deposited on the substrate, the source is quickly coated causing process drift and arcing. In semiconductor batch applications, an etch process is run after set intervals to clean the exposed electrode(s). In continuous processes, such as roll to roll web or in-line coating systems, a PECVD process must run for many 10's of hours without stopping. In these applications an etch cleaning cycle is not practical. Therefore, a linear plasma source is needed that maintains stable operation over long continuous process runs.
Prior art relevant to the present invention can be grouped into three categories: Unbalanced magnetron sputtering sources, hollow cathode sputtering sources and plasma and ion sources.
Unbalanced Magnetron Sources
Window and Savvides presented the concept of unbalanced magnetron (UBM) sputter cathodes in several published articles. In these articles, a Type II unbalanced magnetron is disclosed with its ability to ionize the sputtered flux from the cathode. The fundamental operating principles of the null magnetic field region and mirror magnetic confinement electron trapping are taught.
A planar target type II UBM as presented by Window and Savvides is shown as prior art in FIG. 13. Window and Harding later disclosed a type II UBM without a central magnetic material or high permeability pole. In FIG. 13, magnets 200 are configured around the periphery of a rectangular or round shunt plate 201. Central soft iron pole 207 is located in the center of the shunt plate 201. Due to the unequal magnetic strengths of the peripheral and center poles, a null field point 203 is created above magnetron trap 205 and strengthening field lines above the null point produce a mirror confinement region 208. In operation, magnetron plasma 204 sputters target 206. Electrons leaving the magnetron plasma are trapped in the mirror containment region 208 creating a second visible plasma region. As presented in the literature, the mirror plasma region ionizes a significant portion of the sputter flux from the target. Plasma 208 generated in the mirror region also projects out to substrate 209 and usefully bombards the growing sputtered film. Plasma 208 can be used for plasma processes such as PECVD, plasma treatment etc. While finding use in plasma source applications, the sputtered flux from target 206 is not always welcome, the UBM must operate in the mTorr range typical for magnetron sputtering and, for PECVD applications, and exposed target 206 is quickly contaminated by condensing PECVD constituents.
Hollow Cathode Sputter Sources
The term Hollow Cathode has been used to describe a variety of sputter sources in the prior art.
Rust in U.S. Pat. No. 4,915,805 discloses a hollow cathode confined magnetron with the substrate passing through the center of the cavity.
Sebastiano et al. in U.S. Pat. No. 4,933,057 discloses a hollow cathode configured magnetron with an anode positioned opposite from the opening into the process chamber. The anode in this position will allow electrons to reach the anode without having to pass out of the discharge cavity first. No gas is introduced into the discharge cavity separate from the opening to the process chamber.
Hedgcoth in U.S. Pat. No. 5,073,245 teaches a sputter source in a cavity separate from the process chamber. The magnetic field is along the axis of the cavity cylinder and a magnetron type containment region is reported to be created around the inside of the cavity cylinder walls. The opening to the process chamber creates a discontinuity in the magnetron racetrack. Anodes are located inside the cavity, at each end.
Kubo et al. in U.S. Pat. No. 5,334,302 discloses a sputtering apparatus comprised of multiple magnetron cathode cavities. Process gas is introduced into the base of each cavity. The cavities are open to the process chamber.
Helmer et al. in U.S. Pat. No. 5,482,611 discloses an unbalanced magnetron sputter cathode with a cup shaped or annular cathode. A null magnetic field point is produced adjacent to the cathode opening. The discharge cavity is open to the process chamber. In FIG. 6 of this patent a separate microwave applicator is fitted over the cathode opening. Though separate from the cathode, the applicator opening dimensions are equal to or larger than the cathode cavity. In one embodiment process gas is introduced into the cavity at the base of the cavity opposite the process chamber opening.
Scherer in U.S. Pat. No. 5,728,280 teaches an apparatus for coating substrates by cathode sputtering with a hollow target. The magnetron discharge in the cavity is balanced such that a weak null point is produced well outside the cavity in the process chamber.
Bardos et al. in U.S. Pat. No. 5,908,602 teaches a linear arc discharge source. The discharge cavity does not include a magnetron confined plasma region and the discharge cavity opening is exposed to the process chamber.
McLeod in U.S. Pat. No. 6,444,100 discloses a box shaped hollow cathode sputter source. The bottom of the box is either electrically floating or connected to the cathode. The box is open to the process chamber and process gas is not introduced into the box other than via the process chamber opening.
Other Plasma Sources
Maschwitz et al in U.S. Pat. No. 6,444,945 teaches a bipolar plasma source, plasma sheet source, and effusion cell utilizing a bipolar plasma source. In the preferred embodiment, a magnetron cathode plasma is not created and the hollow cathode cavity opening is exposed to the process chamber.
Miljevic in U.S. Pat. No. 4,871,918 discloses a hollow-anode ion-electron source comprising a discharge cavity with a reduced dimension opening conduit to the process chamber. There is no magnetron confined region or null magnetic field point within the discharge cavity.
Khominich in U.S. Pat. No. 6,103,074 teaches a cathode arc vapor deposition method and apparatus that implements a cusp magnet field. There is no magnetron confined region inside the discharge cavity and the cavity is open to the process chamber.